1. Field of the Invention
The present invention relates to an apparatus and method for preventing reverse power flow of an over current relay (OCR), and more particularly, to an apparatus and method for preventing reverse power flow of an over current relay, which can prevent a protection relay from malfunctioning due to reverse power flow in a distribution system.
2. Discussion of Related Art
Generally, when a line fault or load unbalance occurs in a line interconnected with a distributed generation, a substation circuit breaker, a recloser of a distribution line, an over current relay (OCR) or an over current ground relay (OCGR) of a client circuit breaker malfunctions due to reverse power flow caused by a connection (Y-Δ or Y-Δ-Y) of a transformer.
This way of transformer connection makes the system analysis complicated and causes “false FI information” to be displayed on a screen of a distribution automation system when a distribution line fault occurs, which results in various problems such as a mistake in operation or a delay of fault recovery time.
In order to detect the reverse power flow, a directional over current relay (DOCR) which detects both a voltage and a current has to be installed instead of a protection relay which detects only a current. The directional over current relay detects the direction of power flow to prevent the protection relay from malfunctioning when the reverse power flow occurs.
However, a recloser or a circuit breaker installed in a distribution system which is in general use is a protection device which detects only the current through a current transformer and does not have an algorithm for detecting the reverse power flow, and a method of additionally detecting the voltage is technically impossible.
The substation circuit breaker, the recloser of the distribution line, and a client circuit breaker may malfunction due to the reverse power flow. In particular, when a distribution system is interconnected with a distributed generation such as a solar photovoltaic power generation or a wind power generation, a Y-Δ connection is applied to an interconnecting transformer, and thus the reverse power flow severely occurs.
The directional over current relay has the function to set manually among the forms of forward direction, reverse direction and directionlessness. When a short circuit fault occurs, the directional over current relay operates normally, but when a ground fault occurs, it operates abnormally because a power flow direction changes with respect to the position of ground fault of each phase.
When a fault occurs at a power source side, a person in charge has to always stay in a substation due to the malfunction of the protection relay, or when a fault occurs at a load side in the distribution automation system, much time and manpower are required to find the fault position.
In particular, when a ground fault occurs in the same bank of the substation during distribution system operation, the substation circuit breaker disposed in a neighboring line, the recloser of the distribution line, the ground installed in a client circuit breaker and the current relay cannot detect the directivity by the reverse power flow and frequently malfunctions, whereby the power failure area increases.